There are many types of bicycle locks, ranging from cables, chains, U-locks and folding locks.
Cable locks are very light and inexpensive but provide minimal security against theft, since they are easily cut or broken.
Chain locks have metal chain links covered by a protective sleeve and are locked by a padlock. Portable chains can be wrapped around the seat post.
U-locks are lighter than chains and easily transported by affixing to a carrier mounted on the bicycle frame.
Chain links and U-lock shackles with diameters of less than about 13 mm can be cut with medium-sized bolt cutters. Larger diameters provide better protection against bolt cutters, but have the disadvantage of higher weight and price.
Folding locks are made up of a series of metal bars (flat plate bar links), which are linked together by joints, typically rivets. The rivets allow the bar links to rotate so they can be folded into a compact configuration for storage on the bicycle and folded out for fastening the bicycle to an object.
The weakest part of the folding lock is the joint: bolt cutters can cut through the rivets.